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<title>Diagnostics (The C Preprocessor)</title>

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<h2 class="chapter" id="Diagnostics-1"><span>5 Diagnostics<a class="copiable-link" href="#Diagnostics-1"> &para;</a></span></h2>
<a class="index-entry-id" id="index-diagnostic"></a>
<a class="index-entry-id" id="index-reporting-errors"></a>
<a class="index-entry-id" id="index-reporting-warnings"></a>

<a class="index-entry-id" id="index-_0023error"></a>
<p>The directive &lsquo;<samp class="samp">#error</samp>&rsquo; causes the preprocessor to report a fatal
error.  The tokens forming the rest of the line following &lsquo;<samp class="samp">#error</samp>&rsquo;
are used as the error message.
</p>
<p>You would use &lsquo;<samp class="samp">#error</samp>&rsquo; inside of a conditional that detects a
combination of parameters which you know the program does not properly
support.  For example, if you know that the program will not run
properly on a VAX, you might write
</p>
<div class="example smallexample">
<div class="group"><pre class="example-preformatted">#ifdef __vax__
#error &quot;Won't work on VAXen.  See comments at get_last_object.&quot;
#endif
</pre></div></div>

<p>If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with &lsquo;<samp class="samp">#error</samp>&rsquo;.  For example,
</p>
<div class="example smallexample">
<pre class="example-preformatted">#if !defined(FOO) &amp;&amp; defined(BAR)
#error &quot;BAR requires FOO.&quot;
#endif
</pre></div>

<a class="index-entry-id" id="index-_0023warning"></a>
<p>The directive &lsquo;<samp class="samp">#warning</samp>&rsquo; is like &lsquo;<samp class="samp">#error</samp>&rsquo;, but causes the
preprocessor to issue a warning and continue preprocessing.  The tokens
following &lsquo;<samp class="samp">#warning</samp>&rsquo; are used as the warning message.
</p>
<p>You might use &lsquo;<samp class="samp">#warning</samp>&rsquo; in obsolete header files, with a message
directing the user to the header file which should be used instead.
</p>
<p>Neither &lsquo;<samp class="samp">#error</samp>&rsquo; nor &lsquo;<samp class="samp">#warning</samp>&rsquo; macro-expands its argument.
Internal whitespace sequences are each replaced with a single space.
The line must consist of complete tokens.  It is wisest to make the
argument of these directives be a single string constant; this avoids
problems with apostrophes and the like.
</p>
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